I'm doing this in two different ways on two different BI systems. At my mums house, I use Voice Monkey because It's very simple to set up for a simple Alexa announcement that there is motion. "Someone is at the front door" when there is a person at the front of the house/driveway.
On my personal setup at home, I wanted a more complex and ultimately much more customizable setup where I could have many rules/flows from different cameras that trigger different announcements along with turning lights on/off for set times and other fun stuff. For that, I've got Home Assistant running with a few add-ons installed. MQTT, NodeRed, Alex MeidaPlayer to name a few.
I send all the information I want from BI to Home Assistant using different MQTT using triggers from my motion alerts, I include things like camera name, alert image, what was detected (person/car/dog etc) the AI confidence %, current trigger state etc - I then process those MQTT messages with a mix of standard HA automations and for more complex rules Node Red handle them. Depending on my rules/processing, both HA and Node Red have the ability to send custom text and/or sound to Alexa to perform announcements to one or multiple Echo's in my home. I have some rules that also display the alert images on my Alex Show and Google Nest Hub. The sky is the limit, really. - Worth adding, this is all performed on my local network, nothing in this flow needs Internet/Cloud services.
